Concrete: Properties, Mixing Methods, and Calculations

1. Introduction to Concrete

Concrete is a composite material widely used in construction for its strength, durability, and versatility. It consists mainly of cement, water, fine aggregates (sand), coarse aggregates (gravel or crushed stone), and sometimes admixtures. Its widespread use is due to its adaptability to various forms and strength requirements.

2. Properties of Concrete

Concrete’s behavior is influenced by its constituents and mix proportions. The key properties include:

2.1. Fresh Concrete Properties

Workability: The ease with which concrete can be mixed, placed, compacted, and finished. Slump test is commonly used to measure workability.
Consistency: Related to the fluidity of fresh concrete, depending on the water content.
Segregation: Separation of aggregate from the mix due to improper handling or excess water.
Bleeding: Water rising to the surface after compaction. Controlled using proper mix design and admixtures.
Setting Time: Time for the concrete to transition from plastic to hardened state. Initial setting starts around 30–90 minutes.

2.2. Hardened Concrete Properties

Compressive Strength: Most crucial property. Standard tests are done at 7, 14, and 28 days (e.g., ASTM C39).
Tensile Strength: About 10–15% of compressive strength. Measured by split-cylinder test.
Modulus of Elasticity: Typically ranges from 20 to 40 GPa.
Durability: Resistance to weather, chemicals, abrasion, and other environmental actions.
Shrinkage and Creep: Time-dependent deformations due to moisture loss and sustained loading.
Density: Generally between 2200–2500 kg/m³ for normal-weight concrete.

3. Components of Concrete Mix

3.1. Cement

Acts as the binder. Common types:

OPC (Ordinary Portland Cement)
PPC (Portland Pozzolana Cement)
SRC (Sulphate Resisting Cement)

3.2. Water

Triggers the chemical reaction with cement (hydration). Water-cement ratio (w/c) is critical to strength and workability.

3.3. Fine Aggregates

Usually natural sand or crushed stone. Must be clean and well-graded.

3.4. Coarse Aggregates

Gravel, crushed stone, or recycled concrete. Size typically ranges from 10 mm to 40 mm.

3.5. Admixtures

Additives used to enhance properties:

Plasticizers/Superplasticizers: Improve workability.
Retarders/Accelerators: Control setting time.
Air-Entraining Agents: Enhance freeze-thaw resistance.

4. Concrete Mix Design

The process of selecting suitable ingredients and determining their relative quantities to produce concrete with the required performance.

4.1. Methods of Mix Design

Nominal Mix: Prescribed ratios like 1:2:4 (Cement:Sand:Aggregate). Used in small works.
Design Mix: Based on laboratory tests to achieve required strength with economy. Widely used in structural applications.

4.2. Target Mean Strength

To ensure minimum strength is achieved: f_{target} = f_{ck} + k \times s

= target strength at 28 days
= characteristic strength
= risk factor (typically 1.65 for 5% defect)
= standard deviation (typically 4–5 N/mm²)

5. Concrete Mixing Methods

5.1. Hand Mixing

Used for small-scale jobs. Components are mixed manually on a platform. Accuracy is low, and quality is inconsistent.

5.2. Machine Mixing

Uses concrete mixers (drum or pan type). Improves uniformity and speed.

5.3. Ready-Mix Concrete (RMC)

Prepared in batching plants and delivered via transit mixers. Ensures consistent quality and saves labor.

5.4. Batching

Two methods:

Volume Batching: Measures by volume (less accurate).
Weight Batching: Measures by weight (preferred in design mix).

6. Water-Cement Ratio and Its Importance

One of the most crucial factors in determining strength and durability.

Lower w/c ratio = higher strength and durability.
Higher w/c ratio = increased workability but reduced strength.

Typical range: 0.35 to 0.6

7. Workability and Slump Test

7.1. Slump Test

Measures the consistency of fresh concrete:

True slump: Vertical settlement – desirable
Shear slump: Indicates lack of cohesion
Collapse slump: Too wet

Typical slump values:

25–50 mm: Very low (roadwork)
75–100 mm: Medium (reinforced slabs)
125–150 mm: High (columns, pumped concrete)

8. Concrete Strength Calculations

8.1. Compressive Strength Test

Cube or cylinder samples tested at 28 days: f_{ck} = \frac{P}{A}

= Load at failure (N)
= Cross-sectional area (mm²)

8.2. Volume of Concrete Calculation

For a slab, beam, or column: \text{Volume} = \text{Length} \times \text{Width} \times \text{Height}

8.3. Material Estimation

Let’s take M25 concrete (1:1:2 ratio approx. by volume) as an example.

Cement = 1 Part
Sand = 1 Part
Aggregate = 2 Parts
Water-Cement Ratio = 0.5

For 1 m³ of concrete:

Total = 1 + 1 + 2 = 4 parts
Cement = 1/4 × 1 m³ = 0.25 m³
Convert to kg:
0.25 × 1440 = 360 kg
Water = 0.5 × 360 = 180 kg
Sand = 0.25 m³
Aggregate = 0.5 m³

Add 52–54% for dry volume due to bulking: Total Dry Volume = 1 × 1.54 = 1.54 m³

Recalculate:

Cement = (1/4) × 1.54 × 1440 = 554 kg
Water = 0.5 × 554 = 277 L
Sand = (1/4) × 1.54 = 0.385 m³
Aggregate = (2/4) × 1.54 = 0.77 m³

9. Grades of Concrete

Common grades and their applications:

M5–M10: For non-structural works
M15–M20: For slabs, beams in residential
M25–M30: For columns, beams in commercial
M35+: For bridges, high-rise buildings

10. Curing of Concrete

Curing maintains moisture and temperature for hydration. Methods include:

Water curing: Sprinkling or immersion
Membrane curing: Plastic sheets, curing compounds
Steam curing: For precast units

Curing should continue for at least 7–14 days. Extended curing improves strength and durability.

11. Durability and Quality Control

Durability factors:

Low permeability
Adequate cover
Proper compaction
Use of supplementary cementitious materials (fly ash, silica fume)

Quality control tests:

Slump test
Compression test
Air content
Temperature monitoring

12. Special Types of Concrete

12.1. High-Strength Concrete

Strength above 50 MPa
Used in tall buildings and infrastructure

12.2. Self-Compacting Concrete (SCC)

Flows under its weight
No vibration needed
Ideal for complex forms

12.3. Lightweight Concrete

Lower density (800–1800 kg/m³)
Used for insulation and reducing dead load

12.4. Fiber-Reinforced Concrete

Contains steel, glass, or synthetic fibers
Improved toughness and crack resistance

13. Common Concrete Problems and Remedies

14. Environmental Impact of Concrete

Concrete contributes to CO₂ emissions mainly from cement. Solutions:

Use SCMs like fly ash, GGBS
Recycled aggregates
Carbon-capture cement technologies

15. Conclusion

Concrete is a fundamental material in modern construction, combining simplicity in composition with complexity in behavior. Understanding its properties, mix design, and calculations ensures high-quality, durable, and efficient structures. As construction demands evolve, sustainable and high-performance concrete technologies will lead the industry forward.